A standard has been developed and adopted, directed to a high speed data channel, its associated digital hierarchy, as well as associated optical interface rates and formats for use therein known as the American National Standards Institute, Inc's. "Digital Hierarchy Optical Interface Rates and Format Specification", document T1.105-1988. This document is hereby incorporated by reference. This standard describes the base rate and format along with a multiplexing scheme which defines a modular family of rates and formats available for use in optical interfaces. This standard defines what is called a syychronous transport signal (STS) operating at level one which is the basic logical building block signal that defines all other STS levels greater than one. Within the STS basic signal, is a byte interleave multiplexing scheme with the basic signal divided into a transport overhead section and a portion which contains a payload, which typically contains lower speed telephonic communication channels and their associated path overhead.
Although the synchronous optical network standard has been defined, devices which interface with it in an overall transmission system have, to date, not been implemented. The present invention is directed to such a transmission system, and in particular defines three basic devices; a fiber transmission system, a terminal multiplexer and an add/drop multiplexer. Each device is able to interface with a high speed signal conforming to the SONET standard and is particularly suited for interfacing such high speed signals to typically lower speed signals of either a different standard, such as the digital signal standards (DS-0, DS-1, DS-2, DS-3, etc.) or lower speed SONET signals such as an STS-1 signal.